late 14c., "vapor, odorous vapor; exhalation," from Old French fum "smoke, steam, vapor, breath, aroma, scent" (12c.), from Latin fumus "smoke, steam, fume, old flavor" (source also of Italian fumo, Spanish humo), from PIE *dheu- (1) "dust, vapor, smoke; to rise in a cloud, to fly about (like dust)" (source also of Sanskrit dhumah, Old Church Slavonic dymu, Lithuanian dumai, Old Prussian dumis "smoke," Middle Irish dumacha "fog," Greek thymos "spirit, mind, soul"). In medieval physiology, an "exhalation" of the body that produces emotions, dreams, sloth, etc; later especially of smokes or vapors that go to the head and affect the senses with a narcotic or stifling quality.